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HTTPS是如何保证安全的？</a></li><li><a href="/base/http/WebSocket.html" class="sidebar-link">面试官：说说对WebSocket的理解？应用场景？</a></li><li><a href="/base/http/UDP_TCP.html" class="sidebar-link">UDP、TCP区别</a></li><li><a href="/base/http/OSI.html" class="sidebar-link">面试官：如何理解OSI七层模型?</a></li><li><a href="/base/http/TCP_IP.html" class="sidebar-link">面试官：如何理解TCP/IP协议?</a></li><li><a href="/base/http/DNS.html" class="sidebar-link">面试官：DNS协议 是什么？说说DNS 完整的查询过程?</a></li><li><a href="/base/http/CDN.html" aria-current="page" class="active sidebar-link">面试官：如何理解CDN？说说实现原理？</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/base/http/CDN.html#一、是什么" class="sidebar-link">一、是什么</a></li><li class="sidebar-sub-header"><a href="/base/http/CDN.html#二、原理分析" class="sidebar-link">二、原理分析</a></li><li class="sidebar-sub-header"><a href="/base/http/CDN.html#三、总结" class="sidebar-link">三、总结</a></li><li class="sidebar-sub-header"><a href="/base/http/CDN.html#参考文献" class="sidebar-link">参考文献</a></li></ul></li><li><a href="/base/http/GET_POST.html" class="sidebar-link">面试官：说一下 GET 和 POST 的区别？</a></li><li><a href="/base/http/headers.html" class="sidebar-link">面试官：说说 HTTP 常见的请求头有哪些? 作用？</a></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Linux</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="面试官-如何理解cdn-说说实现原理"><a href="#面试官-如何理解cdn-说说实现原理" class="header-anchor">#</a> 面试官：如何理解CDN？说说实现原理？</h1> <p><img src="https://static.vue-js.com/437ae0f0-b86b-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <h2 id="一、是什么"><a href="#一、是什么" class="header-anchor">#</a> 一、是什么</h2> <p>CDN (全称 Content Delivery Network)，即内容分发网络</p> <p>构建在现有网络基础之上的智能虚拟网络，依靠部署在各地的边缘服务器，通过中心平台的负载均衡、内容分发、调度等功能模块，使用户就近获取所需内容，降低网络拥塞，提高用户访问响应速度和命中率。<code>CDN</code> 的关键技术主要有内容存储和分发技术</p> <p>简单来讲，<code>CDN</code>就是根据用户位置分配最近的资源</p> <p>于是，用户在上网的时候不用直接访问源站，而是访问离他“最近的”一个 CDN 节点，术语叫<strong>边缘节点</strong>，其实就是缓存了源站内容的代理服务器。如下图：</p> <p><img src="https://static.vue-js.com/4f0289f0-b86b-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <h2 id="二、原理分析"><a href="#二、原理分析" class="header-anchor">#</a> 二、原理分析</h2> <p>在没有应用<code>CDN</code>时，我们使用域名访问某一个站点时的路径为</p> <blockquote><p>用户提交域名→浏览器对域名进行解释→<code>DNS</code> 解析得到目的主机的IP地址→根据IP地址访问发出请求→得到请求数据并回复</p></blockquote> <p>应用<code>CDN</code>后，<code>DNS</code> 返回的不再是 <code>IP</code> 地址，而是一个<code>CNAME</code>(Canonical Name ) 别名记录，指向<code>CDN</code>的全局负载均衡</p> <p><code>CNAME</code>实际上在域名解析的过程中承担了中间人（或者说代理）的角色，这是<code>CDN</code>实现的关键</p> <h4 id="负载均衡系统"><a href="#负载均衡系统" class="header-anchor">#</a> 负载均衡系统</h4> <p>由于没有返回<code>IP</code>地址，于是本地<code>DNS</code>会向负载均衡系统再发送请求  ，则进入到<code>CDN</code>的全局负载均衡系统进行智能调度：</p> <ul><li><p>看用户的 IP 地址，查表得知地理位置，找相对最近的边缘节点</p></li> <li><p>看用户所在的运营商网络，找相同网络的边缘节点</p></li> <li><p>检查边缘节点的负载情况，找负载较轻的节点</p></li> <li><p>其他，比如节点的“健康状况”、服务能力、带宽、响应时间等</p></li></ul> <p>结合上面的因素，得到最合适的边缘节点，然后把这个节点返回给用户，用户就能够就近访问<code>CDN</code>的缓存代理</p> <p>整体流程如下图：</p> <p><img src="https://static.vue-js.com/588d7890-b86b-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <h4 id="缓存代理"><a href="#缓存代理" class="header-anchor">#</a> 缓存代理</h4> <p>缓存系统是 <code>CDN</code>的另一个关键组成部分，缓存系统会有选择地缓存那些最常用的那些资源</p> <p>其中有两个衡量<code>CDN</code>服务质量的指标：</p> <ul><li>命中率：用户访问的资源恰好在缓存系统里，可以直接返回给用户，命中次数与所有访问次数之比</li> <li>回源率：缓存里没有，必须用代理的方式回源站取，回源次数与所有访问次数之比</li></ul> <p>缓存系统也可以划分出层次，分成一级缓存节点和二级缓存节点。一级缓存配置高一些，直连源站，二级缓存配置低一些，直连用户</p> <p>回源的时候二级缓存只找一级缓存，一级缓存没有才回源站，可以有效地减少真正的回源</p> <p>现在的商业 <code>CDN</code>命中率都在 90% 以上，相当于把源站的服务能力放大了 10 倍以上</p> <h2 id="三、总结"><a href="#三、总结" class="header-anchor">#</a> 三、总结</h2> <p><code>CDN</code> 目的是为了改善互联网的服务质量，通俗一点说其实就是提高访问速度</p> <p><code>CDN</code> 构建了全国、全球级别的专网，让用户就近访问专网里的边缘节点，降低了传输延迟，实现了网站加速</p> <p>通过<code>CDN</code>的负载均衡系统，智能调度边缘节点提供服务，相当于<code>CDN</code>服务的大脑，而缓存系统相当于<code>CDN</code>的心脏，缓存命中直接返回给用户，否则回源</p> <h2 id="参考文献"><a href="#参考文献" class="header-anchor">#</a> 参考文献</h2> <ul><li>https://zh.wikipedia.org/wiki/內容傳遞網路</li> <li>https://juejin.cn/post/6844903890706661389#heading-5</li> <li>https://blog.csdn.net/lxx309707872/article/details/109078783</li></ul></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">更新时间:</span> <span class="time">12/14/2021, 10:12:35 AM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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